1. Field of the Invention
This invention relates to the operation of polyphase switched reluctance machines by using unconventional patterns of excitation applied to the phase windings.
2. Description of Related Art
In general, a reluctance machine is an electrical machine in which torque is produced by the tendency of its movable part to move into a position where the reluctance of a magnetic circuit is minimized, i.e. where the inductance of the exciting winding is maximized. In one type of reluctance machine, the energization of the phase windings occurs at a controlled frequency. This is generally referred to as a synchronous reluctance machine, and it may be operated as a motor or a generator. In a second type of reluctance machine, circuitry is provided for detecting the angular position of the rotor and energizing the phase windings as a function of the rotor position. This is generally known as a switched reluctance machine and it may also be a motor or a generator. The characteristics of such switched reluctance machines are well known and are described in, for example, "The Characteristics, Design and Application of Switched Reluctance Motors and Drives" by Stephenson and Blake, PCIM '93, Nurnberg, 21-24 June 1993, which is incorporated herein by reference. The present invention is generally applicable to switched reluctance machines operating as motors or generators.
FIG. 1 shows the principal components of a typical switched reluctance drive system. The input DC power supply 11 can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply 11 is switched across the phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive. A rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor. The output of the rotor position detector 15 may also be used to generate a speed feedback signal.
The rotor position detector 15 may take many forms; for example it may take the form of hardware, as shown schematically in FIG. 1, or of a software algorithm which calculates the position from other monitored parameters of the drive system, as described in European Patent Application No. 0573198 (Ray), which is incorporated herein by reference. In some systems, the rotor position detector 15 can comprise a rotor position transducer that provides output signals that change state each time the rotor rotates to a position where a different switching arrangement of the devices in the power converter 13 is required.
The energization of the phase windings in a switched reluctance machine depends on the angular position of the rotor. This may be explained by reference to FIGS. 2(a), 2(b) and 3, which illustrate the switching of the phase windings of a reluctance machine operating as a motor. FIGS. 2(a) and 2(b) show a cross-section of a prior art, 3-phase, switched reluctance machine in which the stator has six poles and the rotor has four poles. Each stator pole 21 carries a coil 23. Coils on diametrically opposite poles are connected in series or parallel to form a phase winding 16. The stator poles in FIGS. 2(a) and 2(b) are labelled to illustrate the three phases A, B & C. The rotor poles 24 protrude from a rotor body 26. The rotor is mounted on a shaft 28 to rotate coaxially in the bore of the stator. Typically, both stator and rotor are formed by stacking laminations of suitable electrical sheet steel and securing them together in known ways. Other forms of switched reluctance machines are well-known in the art: inverted machines have an outer rotor rotating around an inner stator; linear machines have longitudinal stator and rotor members which result in the linear motion of the "rotor".
FIG. 2(a) shows a rotor position where a rotor pole axis is coincident with the stator pole axis of Phase C. This is known as an aligned position for Phase C and is a position where the reluctance of the magnetic circuit associated with Phase C is at a minimum, i.e. the inductance of the phase is a maximum. FIG. 2(b) shows the position where the rotor has been rotated so that the centerline of an interpolar gap is aligned with the stator poles of Phase C. In this position, the reluctance of the magnetic circuit associated with Phase C is at a maximum, i.e. the inductance of the phase is a minimum. As the rotor rotates, the inductance of each phase winding varies cyclically, with the period of the cycle corresponding to the rotor pole pitch.
FIG. 3 generally shows typical switching circuitry in the power converter 13 that controls the energization of the phase winding 16. When switches 31 and 32 are closed, the phase winding is coupled to the source of DC power and is energized. Many other configurations of switching circuitry are known in the art: some of these are discussed in the Stephenson & Blake paper cited above.
FIG. 4 generally shows a rotor pole 24 approaching a stator pole 21 according to arrow 22. As discussed above, when the coil 23 around stator pole 21 (which is a portion of the phase winding 16) is energized, a force will be exerted on the rotor, tending to pull rotor pole 24 into alignment with stator pole 21.
When the phase winding of a switched reluctance machine is energized in the manner described above, the magnetic field set up by the flux in the magnetic circuit gives rise to the circumferential forces which, as described, act to pull the rotor poles into line with the stator poles. This force, acting at the radius of the air gap, develops torque on the shaft. A typical set of torque curves for one phase of a switched reluctance machine is shown in FIG. 5. The torque is shown over a complete rotor pole pitch, as the rotor moves from the position where the centerline of an interpolar gap on the rotor is aligned with the centerline of a stator pole (the "unaligned position"), through the position where the centerlines of the rotor and stator poles are aligned (the "aligned position"), to the position where the next interpolar centerline is aligned with the centerline of the stator pole. The torque curve is periodic with rotor pole pitch, and corresponds to the cyclical variation of inductance of the winding associated with the stator pole.
FIG. 5 shows the torque curves for three currents. As is well-known in the art, the magnitude of torque produced is not linearly related to the current, due to, inter alia, the nonlinearity of the magnetic characteristics of the lamination steel. In general terms, the torque at any point is proportional to the rate of change of inductance of the circuit providing the excitation for the magnetic circuit.
In general, the phase winding is energized to effect the rotation of the rotor as follows. At a first angular position of the rotor (called the "turn-on angle", .theta..sub.ON), the controller 14 provides switching signals to turn on both switches 31 and 32. When the switches 31 and 32 are on, the phase winding is coupled to the DC bus, causing an increasing magnetic flux to be established in the machine. The magnetic flux produces a magnetic field in the air gap which acts on the rotor poles to produce the motoring torque. The magnetic flux in the machine is supported by the magneto-motive force (mmf) which is provided by a current flowing from the DC supply through the switches 31 and 32 and the phase winding 16. In some controllers, current feedback is employed and the magnitude of the phase current is controlled by chopping the current by rapidly switching one or both of switches 31 and/or 32 on and off. FIG. 6(a) shows a typical current waveform in the chopping mode of operation, where the current is chopped between two fixed levels. In motoring operation, the turn-on angle .theta..sub.ON is often chosen to be the rotor position where the centerline of an inter-polar space on the rotor is aligned with the centerline of a stator pole, but may be some other angle.
In many systems, the phase winding remains connected to the DC bus (or connected intermittently if chopping is employed) until the rotor rotates such that it reaches what is referred to as the "freewheeling angle", .theta..sub.FW. When the rotor reaches an angular position corresponding to the freewheeling angle (e.g., the position shown in FIG. 4) one of the switches, for example 31, is turned off. Consequently, the current flowing through the phase winding will continue to flow, but will now flow through only one of the switches (in this example 32) and through only one of the diodes 33/34 (in this example 34). During the freewheeling period, the voltage drop across the phase winding, switch and diode is typically small, and the flux remains substantially constant. The circuit remains in this freewheeling condition until the rotor rotates to an angular position known as the "turn-off angle", .theta..sub.OFF, (e.g. when the centerline of the rotor pole is aligned with that of the stator pole). When the rotor reaches the turn-off angle, both switches 31 and 32 are turned off and the current in phase winding 16 begins to flow through diodes 33 and 34. The diodes 33 and 34 then apply the DC voltage from the DC bus in the opposite sense, causing the magnetic flux in the machine (and therefore the phase current) to decrease.
As the speed of the machine rises, there is less time for the current to rise to the chopping level, and the drive is normally run in a "single pulse" mode of operation. In this mode, the turn-on, freewheel and turn-off angles are chosen as a function of, for example, speed and load torque. Some systems do not use an angular period of freewheeling, i.e. switches 31 and 32 are switched on and off simultaneously. FIG. 6(b) shows a typical such single pulse current waveform where the freewheel angle is zero.
It is well-known that the values of turn-on, freewheel and turn-off angles can be predetermined and stored in some suitable format for retrieval by the control system as required, or can be calculated or deduced in real time.
The torque curves in FIG. 5 represent the positive and negative torques developed as the poles approach and leave each other. In a practical drive, some or all of the positive portion of a curve would be used to provide torque in one direction of rotation, and some or all of the negative portion of the curve would be used for rotation in the opposite direction. Since, even at constant current, the torque varies as a function of angle, the torque resulting from the excitation of all the phases (in turn or two or more simultaneously, depending on the excitation pattern chosen) is not smooth, but contains a ripple component. FIG. 7 shows the resultant curve when the top curve of FIG. 5 is commutated from each phase to the adjacent one. This assumes that phases are identical in their torque output and that the current can be removed from one phase and re-applied to the next phase instantaneously. Inspection of FIG. 2 shows that the rotor rotates clockwise for a counterclockwise phase excitation sequence of A B C. This feature is a consequence of the "vernier" action which arises from the dissimilar number of poles on the stator and rotor, and is peculiar to some forms of doubly salient machines.
From FIG. 7, it can be seen that the resultant torque/angle curve is not smooth, but has a cyclic variation ("torque ripple") about a mean value. While in many applications this ripple component is immaterial, there are applications where the torque ripple can adversely influence the load coupled to the shaft.
There have been many attempts to minimize the torque ripple by altering the excitation pattern of the windings. U.S. Pat. No. 5,319,297 to Bahn, for example, incorporated herein by reference, discloses a method of current shaping (i.e. profiling the value of current as the angle changes) to produce smoother torque. However, this and similar methods of excitation control have an impact on the size of switches required in the electronic controller which supplies the excitation, and the increase in switch size which is required can make a major impact on the cost of the drive. Such consequences have been discussed in, e.g., "Computer-Optimised Smooth-Torque Current Waveforms for Switched-Reluctance Motors" Lovatt, H. C. & Stephenson, J. M., IEE Proc. Electr. Power Appl., Vol. 144, No. 5, Sept. 1997, pp. 310-316, which is incorporated herein by reference.
Though such methods of current profiling can reduce the amount of ripple, it is almost impossible for them to reduce it to the very low levels required by some applications. This is because other effects come into play when the profiling is implemented. For example, to take account of the impossibility of removing or applying current instantly, current profiling schemes normally ramp the current down in one phase while ramping it up in a second phase, thus intending to keep the sum of the torques from the two phases to be a constant value. An example of such teaching is "Optimum Commutation-Current Profile on Torque Linearization of Switched Reluctance Motors", Schramm, D. S., Williams, B. W. & Green T. C., Proc. of ICEM 92, 15-17 Sept. 1992, Manchester, UK, Vol. 2, pp. 484-488, which is incorporated herein by reference. In principle this is a suitable scheme, but it requires finer and finer resolution of rotor position with increasing requirement for reduced ripple. This greatly increases the cost of the drive. Further, the simultaneous operation of two phases requires a consideration of the mutual inductances between phases, since both active phases now share parts of the (nonlinear) magnetic circuit and have adjacent poles where leakage flux is passing between them.
The net result is that there is an economic limit to the amount of reduction which can be achieved in torque ripple by current profiling methods, and the residual ripple is characterized by fast transients of torque over a small angular span, generally centered around the position where the phases overlap. This can be unacceptable in some applications.
Detailed measurements of machines excited with individual and simultaneous currents in two phases show that it is possible for the simultaneous excitation of the phases to produce either more or less torque than the sum of the individual excitations, depending on the polarities of excitation. This is due to the interaction of the mutual and main fluxes in the magnetic circuit. What is desirable is an economical and reliable method of removing the unwanted effects of these mutual fluxes and allowing the machine to produce smoother torque. None of the prior art has presented a satisfactory solution to this problem.